This invention relates to semiconductor devices, particularly to field-effect semiconductor devices as typified by the high electron mobility transistor (HEMT), the two-dimensional electron gas (2DEG) diode (diode that utilizes a layer of 2DEG as a current path or channel), and the metal-semiconductor field-effect transistor (MESFET). More particularly, the invention pertains to such field-effect semiconductor devices that are normally off, instead of normally on as in the case of most of such devices known heretofore. The invention also deals with a method of fabricating such normally-off field-effect semiconductor devices.
The HEMT has been known and used extensively which comprises an electron transit layer of undoped semiconducting nitride such as gallium nitride (GaN) grown on a substrate of silicon or sapphire via a buffer, and an electron supply or barrier layer of n-doped or undoped semiconducting nitride such as aluminum gallium nitride (AlGaN) deposited on the electron transit layer. A source, a drain and a gate (Schottky) electrode are disposed on the electron supply layer.
Made from the dissimilar semiconducting materials, the electron transit layer and electron supply layer differ in both band gap and lattice constant. The electron supply layer of AlGaN is greater in bandgap, and less in lattice constant, than the electron transit layer of GaN or the like. By being placed on the electron transit layer having a greater lattice constant, the electron supply layer generates an expansive strain or tensile stress and so undergoes piezoelectric depolarization. The electron supply layer is additionally subject to spontaneous depolarization.
Consequently, due to both piezoelectric and spontaneous depolarizations, the 2DEG layer is created along the heterojunction between the electron supply layer and the electron transit layer. The 2DEG layer provides a channel of very low resistance, or of high electron mobility, for source-drain current flow. This current flow is controllable by a bias voltage applied to the gate electrode.
As heretofore constructed, the HEMTs of the general construction above were mostly normally on; that is, there was a source-drain flow of electrons while no voltage was applied to the gate. The normally-on HEMT had to be turned off using a negative power supply for causing the gate electrode to gain a negative potential. Use of such a negative power supply made the associated circuitry unnecessary complex and expensive. The conventional normally-on HEMTs were rather inconvenient of use.
Attempts have been made to render the HEMT normally off. Among the suggestions heretofore made to this end are:
1. Accommodating the gate in a recess formed in the electron supply layer.
2. Interposing a p-type nitride semiconductor layer between the gate and the electron supply layer (Japanese Unexamined Patent Publication No. 2004-273486).
3. Accommodating the gate in a recess in the electron supply layer via an insulating film of strontium titanate or the like (Japanese Unexamined Patent Publication No. 2006-222414).
4. Partly removing the electron supply layer to expose part of the electron transit layer and placing the gate on the exposed part of the electron transit layer via an insulating film (WO 2003/071607).
The first cited known scheme is designed to weaken the electric field due to the piezoelectric and spontaneous depolarizations adjacent the part of the electron supply layer that has become thin by creation of the recess therein. The weaker electric field is cancelled out by the built-in potential of the device, that is, the potential difference between the gate and the electron supply layer when no bias voltage is being applied to the gate. The result is the disappearance of the 2DEG layer from the neighborhood of the gate. The device is normally off because there is no source-drain current flow while no voltage is being applied to the gate.
However, the HEMT built on this known scheme had a threshold voltage as low as one volt or even less and so was easy to be triggered into action by noise. What was worse, this threshold changed substantively with manufacturing errors in the depth of the recess in the electron supply layer. Another weakness was that there was a relatively large current leakage upon application of a positive voltage to the Schottky gate. Creating the recess in the electron supply layer brought about an additional shortcoming that the resulting thin part of the electron supply layer became incapable of supplying a sufficient amount of electrons to the electron transit layer even when a voltage was applied to the gate in order to turn the device on. The fact that the 2DEG layer was partly insufficient in electron density the turn-on resistance of the device inordinately high.
The provision of the p-type semiconductor layer under the gate according to the second known suggestion above serves to raise the potential of the underlying part of the electron transit layer. The resulting depletion of electrons from under the gate creates a hiatus in the 2DEG layer, making the device normally off.
An objection to this second known suggestion is the difficulty of creating the p-type nitride semiconductor layer of the sufficiently high hole density required. In cases where this requirement was not met, then it became necessary either to make the electron supply layer thinner or, if the electron supply layer was made from AlGaN or aluminum indium gallium nitride (AlInGaN), to lower its aluminum content. The result in either case was a drop in the electron density of the 2DEG layer, which in turn made higher the source-drain turn-on resistance.
The third known solution also seeks to gain normally-off performance by making the electron supply layer thinner under the gate by creating a recess in that layer. By receiving the gate in this recess via the insulating film, the HEMT is saved from an increase in current leakage and made higher in transconductance (symbol gm).
Having a recess in the electron supply layer, this prior art normally-off HEMT possesses the same shortcomings as the first described one. As an additional disadvantage, the insulating film was susceptible to physical defects, particularly when made thinner for a higher transconductance. A defective insulating film was a frequent cause of current leakage, lower antivoltage strength, device destruction, and current collapse. All these results would be avoidable by making the insulating film thicker, but then the device would be inconveniently low in transconductance.
With the gate placed on the exposed part of the electron transit layer via an insulating film as in the fourth known solution above, the 2DEG layer in the electron transit layer is normally absent from under the gate. A problem with this prior art device arose when the gate was excited to turn it on. Its turn-on resistance was higher than the more conventional normally-on HEMT by reason of the absence of the 2DEG layer from under the gate.
The difficulties and inconveniences pointed out hereinbefore in conjunction with the known normally-off field-effect semiconductor devices are not limited to HEMTs alone. Similar problems have attended the conventional endeavors to provide other types of normally-off field-effect semiconductor devices such as MESFETs and 2DEG diodes.